Implantable prostheses, such as vascular grafts, are commonly used in medical procedures. Tubular grafts may be used to replace or repair damaged or diseased blood vessels. The effectiveness of such devices depends on a number of factors. Important among these are the factors that allow prostheses to match the characteristics of the natural body tissue that is being repaired or replaced, or to adequately compensate for any the prosthesis's shortcomings. In addition, prostheses may exceed the performance of the body part being replaced or repaired, in some respects, or provide some auxiliary function not normally associated with the replaced body part to be replaced or repaired but which may be useful in treating disease or injury.
It is well known to use extruded tubes of polytetrafluoroethylene (PTFE) to make vascular grafts. PTFE is suitable as an implantable prosthesis because of its biocompatibility. PTFE tubes, used as vascular grafts, generally exhibit low thrombogenicity. In vascular applications, the grafts are manufactured from expanded polytetrafluoroethylene (ePTFE) tubes. The microporous structure of these tubes permits natural tissue ingrowth once implanted in a living host. This contributes to healing and long-term patency.
Grafts of ePTFE have a fibrous structure with interspaced nodes connected by fibrils. Where the spaces between the node surfaces are large, tissue ingrowth and cell endothelization may be enhanced. But this characteristic also tends to make the graft delicate. So a number of prior art techniques to reinforce such grafts have been proposed.
One approach has been to modify the structure of the extruded PTFE tubing, during formation, to align fibrils, thereby increasing both radial tensile strength as well as axial tear strength. Forming tubular grafts of multiple layers using different material structures where one tubular structure is formed about another can provide the strength of one material with the porosity and endothelization effectiveness of the other material. Combinations of PTFE and textiles or metal mesh, as used in stents, are known.
With regard to ePTFE grafts, it is known to incorporate antimicrobial agents to form a coating composition which may include a biodegradable polymer and anti-microbial agents, for example, chlorhexidine acetate and pipracil. Known stents or vascular prostheses include those with an overlying biodegradable coating layers that contain drugs, such as anti-coagulants or antibiotic substances. Also known are medical implants where an antimicrobial agent is impregnated throughout the material of the implant or in a region near the surface. Silver may be deposited onto the surface of a porous polymeric substrate via beam deposition and then the pores may be filled with a polymeric material that is biodegradable.
Also known is an implantable graft, with inner and outer coaxial tubes with a space between them separated by ribs. The primary and secondary tubular bodies are joined by the ribs and may be bonded or extruded. The spaces between the tubes can be filled with a leak sealing agent or drug.
There is a need in the art for improved methods of manufacturing multi-channel grafts and also for providing such grafts with certain surface characteristics in the wall-channels.
In the prior art, ePTFE based grafts are typically subjected to a sintering process which is generally done at high temperatures (>100°). Such high temperature exposure can reduce or destroy the bioactivity of many bioactive substances. In addition, there is a need to develop graft-based drug delivery technologies that can provide for drug delivery without altering the surface characteristics of ePTFE grafts.